Economical processes for the preparation of styrene are of great commercial importance. Recently, it has been proposed to prepare styrene from toluene by first converting toluene to benzyl acetate or benzyl alcohol, homologating the benzyl acetate and/or benzyl alcohol to beta-phenethyl acetate and/or to beta-phenethyl acetate or the benzyl alcohol to dehydrating the beta-phenyl alcohol to form styrene. (See assignee's copending U.S. applications Ser. No. 850,209, filed Nov. 10, 1977, and Ser. No. 893,452, filed Apr. 4, 1978, and "Living in Twilight of the Petroleum Age," Chemical Week, July 19, 1978, pp. 40-44.) While this process is of considerable interest, a major drawback is the separation of the numerous components found in the homologation effluent. These include beta-phenethyl acetate, beta-phenethyl alcohol, benzyl alcohol, benzyl acetate, toluene, ethers, acetic acid, spent catalyst and the water of reaction. The benzyl acetate is introduced with the feedstock to the homologation, because of the difficulty of completely separating it from the benzyl alcohol. (The benzyl acetate is formed as a primary oxidation product of the toluene and is then partially hydrolyzed to a mixture consisting of benzyl alcohol and benzyl acetate.) Some of the components in the homologation effluent may be readily separated by extraction or distillation; however, others cannot. For example, the relative volatility of benzyl acetate to beta-phenethyl alcohol is less than 1.2 and that of beta-phenethyl alcohol to the beta-phenethyl acetate only about 1.1. Furthermore, these latter two compounds form an azeotrope, thus preventing their complete separation by conventional distillation.
The need to separate beta-phenethyl alcohol from beta-phenethyl acetate was considered particularly imperative because, when beta-phenethyl acetate was exposed to the dehydrating conditions used to convert the alcohol to styrene (conventionally performed in the presence of an alumina catalyst), it was found that the beta-phenethyl acetate was simultaneously converted to styrene and acetic acid. And, subsequently, the acetic acid decomposed to form undesirable products, namely, carbon dioxide and acetone. On the other hand, when pure beta-phenethyl alcohol is exposed to the cracking conditions used to form styrene from the beta-phenethyl acetate, essentially no reaction occurs.
One approach to the aforesaid problem is to attempt to hydrolyze all the beta-phenethyl acetate to the alcohol, thereby eliminating the need to separate the two components prior to the dehydration step. This, too, however, proved unsatisfactory because this hydrolysis reaction is equilibrium-limited and there is no practical means of driving the reaction to completion.